A solar cell provides power by use of electrons excited in a semiconductor material by exposing the semiconductor material to solar ray. The present solar cell uses electrons stable at the bottom of a conduction band due to energy relaxation of excited electrons. In, order to enhance an efficiency of the solar cell, use of hot carriers, that is, electrons before energy relaxation excited by solar ray is regarded promising. A technique of using a wide range of wavelength band included in solar ray by creating an intermediate energy level in a bandgap of a semiconductor material is also regarded promising.
In order to advance the research and development of the solar cells, it is important to measure energy of electrons excited in a semiconductor material by exposing the semiconductor material to solar ray. For example, if it is possible to know about energy of hot carriers excited or how energy of hot carriers is alleviated or relaxed, the development of solar cells using hot carriers is promoted. Alternatively, if it is possible to know about whether electrons are actually excited into an intermediate energy level or actually excited from the intermediate energy level to a conduction band, the development of solar cells using an intermediate energy level is promoted.
Unfortunately, however, a practical technique for measuring energy of electrons in a semiconductor material excited by solar ray has not been developed yet.
In order to measure energy of excited electrons present in a semiconductor material, photoelectrons emitted from the semiconductor material are needed. The energy of electrons in the semiconductor material can be measured by dispersing photoelectrons emitted from the semiconductor material.
The following techniques for photoemission spectroscopy are known.
(X-Ray Excitation Method)
As illustrated in FIG. 1, in the x-ray excitation method, an X ray is irradiated on a semiconductor material to excite electrons. The X ray has high energy and the electrons are excited to a vacuum level or more so that photoelectrons are emitted from the semiconductor material. The photoelectrons have a kinetic energy; and the kinetic energy is measured thereby to find an energy difference between a core level and the vacuum level of electrons present in the semiconductor material. When an X ray is irradiated on a semiconductor material, electrons present in the core are excited.
Since the X ray has a large half width of energy, a relationship between energy of excited electrons and energy of excitation ray cannot be known with an excellent accuracy. Of course, the method cannot be used for measuring energy of electrons excited by solar ray, which are important to the development of solar cells.
(Ultraviolet-Light Excitation Method)
As illustrated in FIG. 2, in the ultraviolet light excitation method, an ultraviolet ray is irradiated on a semiconductor material to excite electrons. An ultraviolet ray with a short wavelength has high energy, and the electrons are excited to a vacuum level or more so that photoelectrons are discharged outside the semiconductor material. The discharged photoelectrons have a kinetic energy, and the kinetic energy is measured thereby to measure energy of the electrons in occupied states of the semiconductor material.
The ultraviolet light excitation method cannot be used for measuring energy of electrons excited by solar ray, which are important to the development of solar cells.
(2-Photon Excitation Method)
As illustrated in FIG. 3, in the 2-photon excitation method, initial excitation is made by a pump light and further excitation is made by a probe light. Electrons are excited by a pump light to a conduction band, and are further excited by a probe light to a vacuum level or more. For the probe light, a harmonic wave of the pump light is used. The 2-photon excitation method needs excitation by a pulse light, and needs a complicated optical system. The 2-photon excitation method cannot be used for measuring energy of electrons excited by solar ray important to the development of solar cells.
Only energy of electrons excited to a vacuum level or more can be measured by any of the above measurement methods. Even if electrons are excited, if energy thereof is at a vacuum level or less, the energy cannot be measured. Most of the electrons excited by solar ray are at a vacuum level or less, therefore energy of the electrons excited by solar ray cannot be measured by any of the above measurement methods.
Patent Document 1 discloses therein a technique for measuring photoelectrons discharged from a substance with a low vacuum level.
With the technique in Patent Document 1, the number of photoelectrons is measured while irradiating an excitation light on hydrogen-terminated diamond. The hydrogen-terminated diamond has a negative electron affinity, and thus photoelectrons are discharged when an excitation light having low energy such as 5 eV to 5.6 eV is irradiated. Actually, the number of photoelectrons is measured while changing a wavelength of an excitation light. With the technique in Patent Document 1, it is turned out that the hydrogen-terminated diamond has a bandgap of 5.5 eV and an intermediate level due to impurities is formed at a lower level by 0.2 eV.